Britain is sitting on a "potentially massive" amount of shale gas
that experts believe could heat every home for at least 100 years.

The British Geological Survey (BGS) is due to report on how much shale gas is under the country within weeks.

Sources close to the report say the current estimate of five trillion cubic feet is "almost certainly" due to be increased.

Dr Nick Riley, of the BGS, said: "We are sitting on potentially a massive resource, but whether we are able to extract it we do not know. We have to do the exploration and then we have to get the consent of the people."

In the Budget last week George Osborne, the Chancellor, signalled the go-ahead for shale gas by promising tax breaks and bribes for communities that allow drilling in their back yard.

Industry insiders say the BGS could report between 1,200 trillion and 1,800 trillion cubic feet of gas under the UK, mostly in northern England. The other main reserve is around the Hampshire basin in the Home Counties, including Berkshire, Sussex and Kent. There are also pockets in central Scotland, Wales and the Midlands. The top estimate would represent sufficient gas to heat UK homes for 1,200 years.

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Usually it is only possible to extract about a third of shale gas deposits.

Even at conservative estimates, that presume just 10 per cent of 1,500 trillion cubic feet of gas is accessible, there would be enough gas to heat our homes for 100 years.

Hydraulic fracturing – or fracking – allows companies to extract previously inaccessible gas from shale by blasting water into the rocks underground.

Britain lifted an 18-month ban on fracking at the end of last year and already companies are gearing up to start drilling. Cuadrilla estimates there could be as much as 200 trillion cubic feet of shale gas in the Bowland shale alone

The exploration company is planning up to 10 wells around Lancashire over the next 18 months. Other companies ready to explore include IGas and Weir Group.

Past data about shale gas are based on basic geology and relatively shallow deposits. The new information from the BGS will be taken from much deeper wells.

Concerns remain around the possibility of triggering earthquakes, after initial fracking caused tremors and water contamination when gases leaked into the water table.

The BGS is also analysing the groundwater in shale gas areas so that when fracking begins it will be possible to tell if drinking water has been contaminated.

Promised Land, a film starring Matt Damon, to be released this month, presents the negative impact shale gas exploration can have, and is expected to add to safety concerns.

Balcombe in West Sussex has already set up a protest group and the campaign group Frack Off warns that other villages in the Home Counties could be in danger. Prof Richard Davies, of the Energy Institute at Durham University, said contamination of water was "extremely unlikely". Prof Davies said the well tops could be as small as a few feet high but there could be thousands.

"We will need hundreds to thousands of these wells to get enough production for this to make a difference." Shale gas is owned by the Crown, and firms would have to pay tax to the Government. But Prof Davies also sounded a note of caution:

"The BGS can say what they like about the rocks under the ground and the gas in it but no one has produced a molecule yet."